1. Field of the Invention
This invention is directed to a high temperature protective coating composition, particularly for the protection of steel elements exposed to high temperatures in the presence of corrosive agents.
2. Description of the Prior Art
High temperature protective coatings of the type addressed herein are mainly used in cases where the basic material of the construction elements made from heat resistant steels and/or alloys is to be protected at temperatures above 600.degree. C. These high temperature protective coatings are designed to slow down the high temperature corrosion caused mainly by sulphur and oil ashes. The high temperature protective coatings are applied directly to the basic material of the construction element. Such high temperature protective coatings are of special importance on construction elements of gas turbines. They are mainly applied to rotating or guide blades as well as to the heat accumulation segments of gas turbines. By preference, an austenitic material based on nickel, cobalt or iron is used for the manufacture of these construction elements. Nickel super alloys are the basic material principally used in the making of gas turbine parts. The high temperature protective coating to be applied consists preferably of alloys containing chromium.
A high temperature protective coating for gas turbine parts has been known from DE-OS No. 28 16 520. The protective coating consists of a matrix, containing 40 to 60 percent in weight nickel, 15 to 30 percent in weight chromium and 3 to 6 percent in weight boron. The stated weights refer to the total weight of the matrix. In addition, 30 to 40 percent per volume chromium boride--with reference to the total volume of the alloy--is dispersed into the protective coating.
The protective action of this high temperature protective coating is based on the fact that when corrosion occurs, corrosion products form in the shape of covering layers which are corrosion resistant and cover the surface uniformly, homogeneously, tightly and lastingly so that the basic material of the parts is protected from further corrosion attacks. These covering layers that contain mainly chromium oxide, are compatible with the protective layer, chemically as well as mechanically, and are insensitive to shocklike thermal and/or mechanical stress.
However, the high temperature protective coating described above has the disadvantage that the chromium containing covering layer is steamed off at temperatures above 900.degree. C. This leads to a rapid consumption of the high temperature protective coating, especially at the temperatures stated above.
For this reason, the problem addressed was to create a high temperature protective coating in such a way that its wear and tear is lastingly stopped, even when temperatures are above 900.degree. C.